mRNA's for those proteins which are synthesized in vivo on membrane bound ribosomes will be translated in vitro. According to the signal hypothesis these proteins should be synthesized in vitro as larger precursors containing an NH2 terminal extenson. Radiosequence analysis will be performed to verify this contention. A few specific examples among secretory-, lysosomal-, peroxisomal-, mitochondrial-, chloroplast-, and membrane proteins will be analyzed. BIBLIOGRAPHIC REFERENCES: Blobel, Gunter (1976) Extraction from free ribosomes of a factor mediating ribosome detachment from rough microsomes, Biochem. Biophys. Res. Comm. 68, 1-7. Blobel, Gunter, and Dobberstein, Bernhard (1975) Transfer of proteins across membranes. I. Presence of proteolytically processed and unprocessed nascent immunoglobulin light chains on membrane-bound ribosomes of murine myeloma, J. Cell Biol. 67, 835-851.